The present disclosure relates to rolled or extruded metal and sheet metal or metal plate processing. More particularly, the present invention relates to a method and apparatus for stretching extruded or sheet metal using a stretcher machine.
A wide variety of manufactured goods contain processed sheet metal. For example, aircraft, automobiles, file cabinets and household appliances, to name only a few, contain sheet metal. The sheet metal is typically purchased directly from steel mills and/or steel service centers, but may be passed through intermediate processors (sometimes referred to as “toll” processors) before it is received by an original equipment manufacturer.
Various methods exist for flattening or stretching sheet metal. Flatness of sheet metal is important because virtually all stamping and blanking operations require a substantially flat sheet. Also, in certain applications, such as in the aerospace industry, residual stress free material is critical. Good surface conditions are also important, especially in applications where the top and/or bottom surfaces of the metal sheet will be painted.
There are a number of common defects that effect sheet metal flatness. For example, when sheet metal is rolled into coil form for convenient storage and transportation, the strip tends to take on a coiled shape. This curvature is commonly referred to as “coil set.” Coil set occurs because the sheet metal has been bent past its yield point. More specifically, when sheet metal is coiled, the metal fibers near the inside surface of the curved sheet are compressed past their yield point, and the metal fibers near the outside surface of the curved sheet are stretched past their yield point. Another type of shape defect known as “edge wave” occurs if the edge portions of the sheet are no longer than the center portion of the sheet, resulting in undulations in one or both of the edge portions of the sheet. A similar type of shape defect known as “center buckle” results if the center portion of the sheet is longer than one or both of the edge portions, which results in bulging or undulating of the central portion of the sheet.
An existing method of flattening sheet metal is called “stretcher leveling.” A conventional C-frame stretcher leveler is shown schematically in FIG. 1. Stretcher leveling is generally considered to be a superior flattening process because, unlike roller leveling and temper processing, it rectifies the problem of internal residual stresses in the sheet metal and produces a flatter product without crown reduction. As shown in FIG. 1, a typical C-frame stretcher leveler includes a pair of generally C-shaped grippers or jaws G that securely grips the opposing ends of the sheet S to be stretched. The surface portions of the grippers that engage or grip the sheet metal to hold the sheet against movement during stretching are typically grooved, knurled or serrated to provide a secure grip. In operation, the grippers G are hydraulically or pneumatically controlled to engage the opposed ends of the sheet S and, once a firm contact is made, hydraulic actuators (not shown) move the grippers in opposite directions from one another to stretch the metal sheet S held therebetween in opposed directions. The entire cross section of the metal sheet is stretched past its yield point (i.e., beyond its elastic limit) such that all internal residual stresses are eliminated from top to bottom and from side to side.
However, a problem with a conventional C-frame stretcher leveler is that it cannot be used with continuous strips of metal because the C-shaped grippers clamp at the opposed ends of a metal sheet, as shown in FIG. 1. Another problem with conventional C-frame stretcher levelers is that the grippers bite deeply into the metal and disfigure the top and bottom surfaces of the sheet. Traditionally, the disfigured portions of the sheet are cut off as scrap, which results in a substantial amount of wasted material. Also, operation of a C-frame stretcher leveler is very labor intensive because the individual sheets must be moved into and out of the machine between operations. Aside from cutting off the disfigured portions, conventional C-frame stretcher levelers do nothing to improve the surface quality of the sheet metal.
U.S. Pat. No. 4,751,838, issued to Kenneth Voges, discloses an “in-line stretcher leveler.” The teachings of this patent are incorporated herein by reference. The basic components of conventional in-line stretcher leveler are shown schematically in FIG. 2. As shown in FIG. 2, a typical in-line stretcher leveler includes a first set of upper and lower gripping members GU1 and GL1 and a second set of upper and lower gripping members GU2 and GL2. The gripping members are hydraulically or pneumatically controlled to engage top and bottom surfaces of the metal sheet S and, once a firm contact is made, hydraulic actuators (not shown) move the first and second sets of gripping member in opposite directions from one another to stretch the segment of the metal sheet S positioned between the two pairs of gripping members. Then, the gripping members are released and the metal sheet S is advanced so that the next section of the metal sheet can be stretched. Because the gripping members of the in-line stretcher leveler engage the metal sheet from the top and bottom, rather than at opposing ends, the in-line stretcher leveler can be used to stretch any length of sheet metal by successive stretching operation. As disclosed in U.S. Pat. No. 4,751,838, unlike the grippers of the C-frame stretcher leveler, the gripping members GU1 and GL1, GU2 and GL2 of the in-line stretcher leveler preferably have engagement surfaces that are sufficiently smooth to avoid marring or otherwise disfiguring the surfaces of the metal sheet S. This is particularly advantageous because there are no disfigured portions to be cut off as scrap, which results in substantial cost savings. Also, this process is far less labor intensive than C-frame stretcher leveling.
A problem with some existing stretchers is that the cam-style jaw has a profile such that the gripping angle changes depending on the thickness of the workpiece. This results in less efficient and unreliable gripping force across a variety of extrusion thicknesses or sizes. Also, the gripping force is disproportional to the stretching force. It is thus desirable to provide a stretcher for extrusions or plates or sheets which provide a uniform or constant gripping angle for various extrusion sizes or thickness and a gripping force which is proportional to the stretching force.